This invention relates generally to a method for determining the rotational velocity of an axle (and in the limiting case of no detected axle speed then to also serve to detect a locked axle condition for a vehicle). More particularly, the invention relates to a method for determining the rotational velocity of an axle and detecting a locked axle condition for an AC locomotive, which is propelled by AC traction motors.
Locomotives used for hauling applications are generally equipped with speed sensors coupled to individual traction motors or to the axles driven by the traction motors. The speed sensor data or speed sensor information is monitored and is used for various function like torque production, speedometer, cruise control etc. It is also used to detect a locked axle condition. If a locked axle condition occurs on a given axle while the locomotive is moving, the respective rotational axle speed will decrease to zero, while the remaining axles will continue to rotate at the speed corresponding to the speed of the locomotive. This locked axle condition will cause the axle wheel to slide along the rail and could adversely affect the handling characteristics of the locomotive. Thus, the speed sensor can usually detect a locked axle condition whenever a substantial difference in speed exists in one axle relative to the remaining axles.
When a locked axle is detected the locomotive operator generally has to stop the locomotive and inspect the axle for any visual damage. If there is no visual damage to the axle, then the locomotive is moved slightly in order to ascertain if the axle is in fact locked or if the detection of the locked axle is a false detection caused by a failure in the speed sensor. Using the current method of detecting a locked axle, when a locked axle condition is detected due to a failure in the speed sensor, the locked axle speed sensor and/or speed detection system is disabled and the locomotive is operated at a lower speed limit until the locked axle speed sensor and/or speed detection system can be repaired.
Unfortunately, this method has a number of disadvantages. One disadvantage is that stopping locomotives, inspecting axles and operating at slower speeds involve burdensome delays and can be expensive. A second disadvantage of this method is that it allows a large amount of electric power to be lost when an axle becomes locked. This is because the traction motor is driven by power supplied to the traction motor by the locomotive. When an axle becomes locked power is still supplied by the locomotive to drive the traction motor and this power becomes lost in the semiconductor devices of the traction motor. A third disadvantage is that if any of the electronic components of the locked axle speed sensor and/or speed detection system fails then locked axle detection is not possible. If the sensors do not function correctly, then a locked axle condition may occur without being detected.
Similarly if the speed sensor connected to the speedometer is not functional, the locomotive may not operational due to potential inability to be operated within the speed limit. Two methods to determine speed and hence locked axle condition of a vehicle having traction motors are known. The first one is a speed sensor, which senses the rotation of the shaft. The second one determines the speed by analyzing the response (for example, current or torque) to an excitation of the motor (for example, voltage). If the speed sensor fails or is not available, the only known method to determine the speed is to excite the motor. In that regard, the source of voltage/power used to excite the traction motors is produced on the locomotive typically by regulating the field of an alternator driven by an engine. There are many operating conditions in a locomotive when this source of voltage is not available like when the engine is not running. Moreover, the application of power to the motor produces losses in the power semiconductor devices and other electrical components, such as the motor and alternator.
More particularly, in the prior art locomotives use traction motors to drive the axles. A speed sensor senses the passing of a tooth gear attached to either the axle shaft or the motor shaft. These speed sensors, while adequate for their intended purposes, can be costly and may experience failures. Therefore, additional speed sensors for redundancy may not be desirable. Locked axle detection also uses the above speed sensor information. Methods of determining locked axle condition when a speed sensor fails are described in U.S. Pat. Nos. 5,990,648 and 6,532,405 also assigned to the assignee of this application. U.S. Pat. No. 5,990,648 describes a technique which can be used for determining locked axle condition when the motor/axle is not producing any useful torque and U.S. Pat. No. 6,532,405 describes a technique which can be used for determining locked axle condition when the motor/axle is producing useful torque (motoring/dynamic braking). In both cases, the traction motor is excited with an AC voltage of known amplitude and frequency. The current response of the motor to this applied voltage and frequency is used to determine the state of the motor.
However there are many operating conditions of a locomotive when an excitation voltage is not available. For example, when the engine is not running, or when the locomotive is in isolated mode such that the alternator cannot produce voltage. Another instance when an excitation voltage is not available is when there is a failure or unavailability of the components used in the production of AC voltage to the motor like inverters and power electronics. Furthermore, a failure in the components used to produce voltage to the inverter like the alternator and its field excitation system or failure in the components attached to the DC traction bus (like dynamic brake grid resistors and contactors or failure in the diesel engine components). During any of these conditions, it will be desirable to know the speed of the motor/axle, as well as detect and verify the existence of a locked axle condition.
Therefore, there is a need for a low-cost method to reliably determine the speed of the motor/axle and to thus determine when a locked axle condition occurs in a vehicle wherein the excitation source is not available or is otherwise not desirable for use and wherein the method does not rely solely on axle speed sensors for locked axle detection.